Method and Device for Continuous Casting of Metals

ABSTRACT

The invention relates to a method for close-to-final-dimension casting of billets made of metal, particularly rectangular billets, wherein molten metal ( 2 ) is cast onto a circulating transport belt ( 3 ), followed by in-line rolling and cooling of said transport belt ( 3 ). The invention also relates to a device for carrying out the inventive method.

The invention concerns a method for the near-net-shape casting of metal strands, especially rectangular strands, where liquid metal is cast on a revolving conveyor belt with subsequent in-line rolling. The invention also concerns a device for carrying out this method.

In near-net-shape strip casting, liquid metal is cast through an opening in the wall of a horizontally movable feed tank onto the upper side of a horizontally revolving belt, where it solidifies. After solidification has occurred, the strip that has been cast in this way is conveyed directly to a rolling stand or a rolling train.

EP 1 077 782 B1 describes a method for the near-net-shape casting of rectangular strands of metal, especially steel, followed by in-line rolling out of the strand, with a material feed tank, through whose discharge nozzle the liquid metal is fed onto the carrying run of a conveyor belt, on which it solidifies and is transferred to a rolling stand, with the following steps:

(a) before the start of casting:

(aa) the point at which the liquid metal is fed onto the conveyor belt is roughly predetermined, and

(ab) the conveying speed of the conveyor belt is set as a function of a desired rolling thickness and rolling speed of the rolling stand.

(b) during casting:

(ba) the position of complete solidification of the metal strand on the conveyor belt is determined,

(bb) the temperature of the rolling stock in the vicinity of the rolling stand is detected, and

(bc) the position of complete solidification and the temperature of the rolling stock are used as control variables for the current position of the point at which the liquid metal leaving the material feed tank is fed onto the conveyor belt.

The cited document also discloses a device for the near-net-shape casting of rectangular strands of metal, especially steel, followed by in-line rolling out of the strand, which device comprises a material feed tank with a discharge nozzle, a horizontally arranged conveyor belt, and at least one rolling stand downstream of the conveyor belt, where the material feed tank is connected with motion control elements for moving it in the horizontal direction, coaxially with the principal axis of the conveyor belt in the same direction as or in the opposite direction from the direction of conveyance of the strand, and where the material feed tank is connected to an actuator, which is automatically controlled by an automatic control system, which is connected to the actuator and to measuring elements for determining the position of the complete solidification of the strand and measuring elements for determining the temperature of the rolling stock.

The prior art thus includes a method and a device in which the point at which the metal is fed onto the conveyor belt is locally fixed or locally variable.

A disadvantage of a locally fixed metal feed point is that it greatly limits the production spectrum. Only products with small variations in dimensions or material properties can be produced. An improvement was realized with a variable liquid metal feed point onto the conveyor belt. However, a method or device of this type has the disadvantage that the cooling is not adapted to the variable boundary conditions. It was realized that the type of cooling and the position or spatial configuration of the cooling during the strip casting affect the heat dissipation, for example, in such a way that local heating of the conveyor belt occurs, which causes the conveyor belt to fail. Furthermore, the effective heat transfer can be so low that sufficient solidification of the cast strip is not achieved.

Therefore, the objective of the invention is to specify a method and a device in which the production window or production spectrum is widened. This includes the casting of different metals and grades, the casting of different production thicknesses and widths, and a wide variance of the casting speed in order to avoid the disadvantages specified above.

In a method of the type specified in the introductory clause of claim 1, the objective of the invention is achieved by cooling the conveyor belt.

Further embodiments of the method are specified in the dependent claims concerning the method.

The invention also concerns a device for carrying out the method of the invention.

Further embodiments of the device are specified in the dependent claims concerning the device.

The decisive advantage of the method of the invention is that the intensity of the cooling is designed according to the greatest heat transfer in such a way that the greatest cooling effect is realized at the point of first contact of the liquid metal with the conveyor belt and decreases downstream. A more flexible production spectrum is realized by local variation of the point at which the liquid metal is fed onto the conveyor belt in conjunction with optimally adapted cooling or cooling configuration.

The point at which the liquid metal comes into contact with the conveyor belt must be varied in the casting direction under certain boundary conditions, such as varying metal grades, mass flow rates and the like. To this end, the intensity of the cooling is adjusted by local variation of the cooling zone in the direction of conveyance. Therefore, the zone of the conveyor belt that has the greatest cooling intensity is correlated with the point of discharge of the liquid metal from the feed tank.

The method of the invention and the device of the invention make the effective cooling length and heat dissipation more flexible to widen the production window. This makes it possible to cast materials that require more or less intense cooling in a wide range of flow rates.

In a first embodiment, the nozzles are combined in several independent units. A separate, pressure-controlled water supply is assigned to each nozzle unit. In a device of this type, the pressure with which the coolant is sprayed against the underside of the carrying run of the conveyor belt is greatest in each case at the point at which the liquid metal is fed onto the upper side of the carrying run of the conveyor belt. In the direction of conveyance, the pressure in the following nozzle units is reduced, e.g., incrementally. By applying the greatest pressure at the point at which the liquid metal is fed onto the conveyor belt, one achieves the greatest cooling effect at that point.

In the first embodiment, the pressure in the individual nozzle units is varied.

In a second embodiment, the pressure with which the individual nozzle units spray the coolant at the underside of the carrying run of the conveyor belt remains constant. In this embodiment, the individual nozzle units are arranged in such a way that the nozzle unit with the greatest cooling effect, i.e., the greatest coolant volume flow rate, is always positioned where the liquid metal is fed onto the conveyor belt. This is accomplished by local shifting or displacement of the nozzle units.

Furthermore, to obtain a solidified strip at the end of the conveyor belt, the parameters of conveyor belt speed and amount of metal/time are varied. The effective cooling length necessary for solidification is adapted to the metallurgical length.

This process is carried out as follows in various situations, and uniform feeding of the liquid metal to the conveyor belt is assumed.

Shortening of the Effective Cooling Length During the Casting Process

Case 1: The relative speed between the unit Z/I and the conveyor belt is held constant. The speed of the conveyor belt v_(Tr) must be raised by the amount of the horizontal speed of the unit Z/I: v _(Tr new) =v _(Tr old) +v _(unit Z/I) where v_(Tr) is the speed of the conveyor belt and v_(unit Z/I) is the speed of the unit Z/I.

The mass flow rate m is held constant—when the end position of the unit Z/I is reached, the conveyor belt speed v_(Tr) is reduced to its original value.

Case 2: The conveyor belt speed v_(Tr) is held constant. The metal feed must be reduced by the amount m=d×b×rho×v _(unit Z/I) in (t/min) where m is the mass flow rate, d is the thickness of the strand, b is the width of the strand, rho is the density of the liquid metal, and v is the speed of the unit Z/I.

When the end position of the unit Z/I is reached, the flow rate m is raised to its original value.

Lengthening of the Effective Cooling Length During the Casting Process

Case 3: The relative speed between the unit Z/I and the conveyor belt is held constant. The speed of the conveyor belt v_(Tr) must be reduced by the amount of the horizontal speed of the unit Z/I: v _(Tr new) =v _(Tr old) −v _(unit Z/I) The mass flow rate m is held constant. When the end position of the unit Z/I is reached, the conveyor belt speed v_(Tr) is raised to its original value.

Case 4: The conveyor belt speed v_(Tr) is held constant. The metal feed must be raised by the amount m=d×b*rho×v _(unit Z/I) (t/min) When the end position of the unit Z/I is reached, the flow rate m is reduced to its original value.

The processes that have been explained are graphically represented below

Example of a typical conveyor belt speed v_(Tr): 40 m/min v_(unit Z/I): 10 m/min

Case 1: _(Tr)=50 m/min

Case 3: v_(Tr)=30 m/min

Example of a typical flow rate of the plant: m=0.012 m×1.3 m×7.6 t/m³×40 m/min=4.7 t/min v _(unit Z/I): 10 m/min→m Δm=1.2 t/min

Case 2: m=3.5 t/min

Case 4: m=5.9 t/min

Specific embodiments of the invention are described in greater detail below with reference to the highly schematic drawings.

FIG. 1 shows a strip casting plant with pressure control of the nozzle segments, where a metal feed tank is located in different positions (1 a, 1 b, 1 c).

FIG. 2 shows a strip casting plant with interchangeable nozzle segments, where a metal feed tank is located in different positions (2 a, 2 b, 2 c).

In FIGS. 1 a, 1 b, 1 c, a metal feed tank 1 for liquid metal 2 is located above a conveyor belt 3. The conveyor belt 3 runs around two rollers 4 and 5. Liquid metal 2 is fed from an opening 6 in the metal feed tank 1 onto the upper side 7 of the carrying run 8 of the conveyor belt 3. Rotary motion of the rollers 4 and 5 causes the liquid metal 2 to be conveyed in conveyance direction 9 to a rolling installation (not shown).

In this regard, the liquid metal 2 must have formed a strand shell of sufficient strength when it leaves the conveyor belt 3 in the area of roller 5. To cool the conveyor belt 3 and thus the liquid metal 2, nozzles 11 are installed near the underside 10 of the carrying run 8 of the conveyor belt 3. A coolant, such as water or the like, is sprayed from the nozzles 11 towards the underside 10 of the carrying run 8.

The nozzles 11 are arranged, for example, in four nozzle segments 12, 13, 14, 15. Each nozzle segment 12, 13, 14, 15 has a separate, pressure-controlled water supply (not shown). This makes it possible for each nozzle segment 12, 13, 14, 15 to be pressurized with a different pressure. The highest pressure of the cooling water or coolant is provided where the greatest amount of heat must be dissipated. This location corresponds to the point at which the liquid metal 2 is fed onto the upper side 7. In FIG. 1 a, this point is located on the left side. Therefore, the nozzle segment 12 is pressurized, for example, with a pressure of 8 bars. Since the amount of heat to be removed decreases in the direction of conveyance 8, nozzle segment 13 is pressurized with a reduced pressure of, for example, 6 bars, nozzle segment 14 with 4 bars, and nozzle segment 15 with 3 bars.

The nozzle segment located upstream of the point at which the liquid metal 2 is fed onto the upper side 7 are also pressurized with a reduced pressure (nozzle segment in FIG. 1 b, and nozzle segments in FIG. 1 c).

The nozzle segment located upstream of the point at which the liquid metal 2 is fed onto the upper side 7 are also pressurized with a reduced pressure (nozzle segment in FIG. 1 b, and nozzle segments in FIG. 1 c).

The pressures can be individually adjusted at any time and are affected by the aforementioned boundary conditions, such as metal properties, mass flow rate, etc.

In the device of the invention which is illustrated in FIGS. 2 a, 2 b, 2 c, the cooling water or coolant is supplied under constant pressure to the individual nozzle segments 16, 17, 18, 19, 20. All of the nozzle segments 16, 17, 18, 19, 20 can be supplied by a centralized system, or each individual nozzle segment can be supplied by a decentralized system. In this connection, the nozzles of the nozzle segments are designed in such a way that the nozzle segments 16, 17, 18, 19, 20 have different cooling effects. This can be achieved, for example, by different volume flows of the coolant.

In accordance with the invention, the nozzle segment 16, 17, 18, 19, 20 with the greatest cooling effect is positioned where the liquid metal 2 is fed onto the conveyor belt 3. Since this place varies, the nozzle segments 16, 17, 18, 19, 20 can be interchanged or shifted. In FIG. 2 a, the greatest cooling effect is achieved in the left nozzle segment 16. The cooling effect decreases in the following nozzle segments 17, 18, 19, 20 in the direction of conveyance 9.

In FIG. 2 b, the point of delivery of the liquid metal 2 is displaced in the direction of conveyance 9. To produce the greatest cooling effect here, the nozzle segment 16 described in connection with FIG. 2 a is likewise displaced in the direction of conveyance 9.

To achieve a uniform gradient in the cooling effect, the downstream nozzle segments 17, 18, 19, 20 are each displaced by one position to the right. A displacement by one additional position is shown in FIG. 2 c.

When the parameters of conveyance speed and amount of metal unit time are varied as described above, the effective ling length is thus adapted to the metallurgical length.

LIST OF REFERENCE NUMBERS

1 metal feed tank

2 liquid metal

3 conveyor belt

4 roller

5 roller

6 opening

7 upper side

8 carrying run

9 direction of conveyance

10 underside

11 nozzles

12-20 nozzle segment 

1. A method for the near-net-shape casting of metal strands, especially rectangular strands, where liquid metal (2) is cast on a revolving conveyor belt (3) with subsequent in-line rolling, wherein the conveyor belt (3) is cooled, the cooling is effected by spraying coolant or cooling water by nozzles (11) onto the underside (10) of the carrying run (8) of the conveyor belt (3), the nozzles (11) are combined to form nozzle segments (12-20), and a separate, pressure-controlled water supply is assigned to each nozzle segment (12-20).
 2. A method in accordance with claim 1, wherein the coolant pressure of the nozzles (11) is different in each nozzle segment (12-15).
 3. A method in accordance with claim 1, wherein the coolant pressure of the nozzles (11) is the same in each nozzle segment (16-20).
 4. A method in accordance with claim 1, wherein the volume flow rate of the coolant or cooling water in the individual nozzle segments (12-20) is varied.
 5. A method in accordance with claim 1, wherein the volume flow rate of the coolant or cooling water in the individual nozzle segments (12-20) is set at a constant level.
 6. A method in accordance with claim 1, wherein the mass flow rate m of the liquid metal is held constant.
 7. A method in accordance with claim 1, wherein the conveyance speed v_(Tr) is held constant.
 8. A method in accordance with claim 1, wherein the metal feed is increased or decreased.
 9. A method in accordance with claim 1, wherein the conveyance speed v_(Tr) is increased or decreased.
 10. A device for the near-net-shape casting of metal strands, especially rectangular strands, where liquid metal (2) is cast on a revolving conveyor belt (3), which runs around two rollers (4, 5), has a carrying run (8) with an upper side (7) and an underside (10), and is followed by a rolling installation, wherein nozzles (11) are arranged in nozzle segments (12-20) below the underside (10), and each nozzle segment (12-20) has a separate, pressure-controlled coolant or cooling water supply.
 11. A device in accordance with claim 10, wherein the nozzles (11) in the individual nozzle segments (12-20) have different volume flow rates.
 12. A device in accordance with claim 10, wherein the nozzles (11) in the individual nozzle segments (12-20) have identical volume flow rates.
 13. A device in accordance with claim 10, wherein the nozzle segments (16-20) are interchangeably arranged. 